Valve arrangement

ABSTRACT

A valve device including a cylindrically-shaped space and a displaceable piston dimensioned to fit and move between forward and rearward stop positions within the space, having a first opening connection or inlet to the hydraulic system and a second opening connection or outlet to the tightening piston in the space and an openable check valve positioned in the outlet such that the check valve allows fluid to flow only in one direction from the cylindrically-shaped space to a hydraulic tightening piston in a sawing gear for adjusting the tightness of a saw chain. The displaceable piston, with the hydraulic system in a non-pressurized state, is urged by the influence of a tensioned spring to its forward stop position opening the check valve and with the hydraulic system in a pressurized state, is pushed back by the pressure in said cylindrically-shaped space to its rearward position which permits the check valve to close preventing hydraulic fluid from flowing from the hydraulic tightening piston and into the cylindrically-shaped space.

BACKGROUND OF THE INVENTION

The present invention concerns a valve arrangement for adjusting ahydraulic saw chain tightening device in sawing gear that includes aholder and a saw bar displaceable in a longitudinal direction relativeto the holder for regulating the tightness of a saw chain running aroundthe saw bar and holder, whereby the saw chain tightening device isconnected by the valve arrangement to a hydraulic circuit system andincludes at least one hydraulic piston affecting the saw bar.

When mechanised means are used to improve the efficiency of loggingoperations, it is preferable to use what is known as a timber cuttingunit comprising a sawing gear that includes a saw chain and a saw bar.Efficient logging puts high demands on the equipment used with regard tosafety, robustness, ease-of-use as well as precision and performance,i.e. that the equipment works quickly.

It is well known that a saw chain that rotates under conditions of highspeed around a sew bar requires chain tightening at relatively shortintervals. In addition, it is also known that the problem of chaintightening can be solved by an automatic and continuous tightening ofthe saw chain, which is achieved by making the saw bar displaceable inthe saw bar's longitudinal direction in relation to the drive gear ofthe saw chain. However, the chain-tightening force must not be toogreat, as too great a tension increases the wear on the chain andshortens the working life of both the saw chain itself and of the wheelaround which the saw chain runs at the free end of the saw bar.

An attachment for a saw bar of a chain saw is described in SE 467,488.The attachment includes plate with a grooved recess in which a steeringpart united with the saw bar moves in the saw bar's longitudinaldirection. A piston projecting from the recess in the plate in thelongitudinal direction of the saw bar bears on the guiding part when inits active operating position. The piston gets its power from a fluidsystem integrated and enclosed in the plate and that includes agas-sprung pressure chamber that is connected to the piston by a lineand a valve. The valve is a manually switchable check valve, that in oneposition permits fluid to flow only from the pressure chamber to thepiston, thus tightening the chain by displacing the saw bar, and in asecond position permits fluid to flow only from the piston to thepressure chamber, which, due to an increase of pressure in the chamber,restores the position of the piston when, for example, replacing the sawchain with a new one.

The technology of an enclosed system carries with it, among otherthings, the limitation that the pressure at the piston varies dependingon the position of the piston. The stretching force is greater when thepiston is in its inner position and somewhat less when the piston is inits outer position. Further disadvantages are the problems of having tomaintain and/or consciously be able to vary the pressure and thus thestretching force over time.

When a new chain is to be fitted, the check valve is switched and alevering tool is used to force back the piston. A high pressure in theenclosed hydraulic system, which is needed to ensure a sufficientlygreat stretching force for the saw chain, means that a powerful force isrequired to push back the piston. This is unsatisfactory, partly becauseof the working environment and the risk of injury to the user, andpartly because loose specialised tools have a tendency to disappear orbe difficult to find when they are needed.

SE 502,386 illustrates how a similar piston is arranged, in this case ina steering part permanently attached to the saw bar and accommodated ina groove in the attachment/holder, whereby the piston, when affected bythe fluid under pressure, keeps the saw bar in a position displaced fromthe attachment/holder. In this case, the piston receives its power froma hydraulic system on the mechanical equipment on which the sawing gearis mounted. An additional pair of pistons are also found on the samehydraulic system arranged to stabilise the saw bar by means of aclamping action across the direction of displacement of the saw bar. Thesaw bar is thus affected partly by a piston arranged to push the bladefrom the holder so that the saw chain is kept tight, and partly by atleast one clamping piston that holds the blade tightly and stabilises itin position. The force keeping the chain tight is somewhat greater thanthe force of friction that results from the clamping pistons so that acontinuous tightening is achieved while the chain still displays arelatively stable attachment. All the pistons are connected to themachinery's hydraulic system and when this equipment is in operation,the pistons are provided with what is essentially a constant pressure.

A check valve is also arranged in the line from the hydraulic system.This check valve is essential to ensure that the saw chain does notbecome slack during operation. This could happen as a result of therapid loading and pressure increases that occur at the piston, forexample, at start-up when the saw chain goes from stationary position tofull speed in a very short time, or when a saw chain moving a high speedis forced against a tree trunk or when a branch gets caught in thechain, etc.

When a saw chain is to be replaced, the pistons must be forced back,which means that the hydraulic fluid behind the pistons must be disposedof. In devices similar to that described in SE 502,386, this is done asfollows. Firstly, the machinery is stopped so that the hydraulic systemloses its pressure. The check valve, however, ensures that the pressureis maintained behind the piston(s), which is why chain replacement callsfor emptying to ease the pressure. In practice, emptying requires thatthe operator needs to have a special tool at hand to open the evacuatingvalve that is normally located on the sawing gear. As a collectionvessel for the fluid is usually not available, this evacuation means anunwanted oil spill in a natural environment. In addition, evacuationalso brings the risk that air may enter the hydraulic system, which ifit happens, will greatly diminish the sought after rigid tightening andstabilising function of the pistons.

SUMMARY OF THE INVENTION

One object of the present invention is to overcome the disadvantagesmentioned above and to achieve a valve device that functions as a checkvalve when the hydraulic system is pressurised, and that automaticallyevacuates the pressure without fluid spillage when the hydraulic systemis not pressurised, thereby making it possible to replace a saw chainwithout the need for any special tool.

This objective is achieved with a valve device of a kind first mentionedabove and which has those features evident from the characteristics ofattached claim 1.

We propose, therefore, a valve device that, when the equipment is inoperation and the hydraulic system pressurized, is by means of thispressure in a condition that renders it able to function as a checkvalve that allows the fluid to flow in a direction towards thepiston(s), and, when the equipment is shut down and the hydraulic systemis accordingly unpressurised, is by means of the absence of thispressure, in a condition that allows the fluid to flow back from thepiston(s).

Further characteristics and advantages of the invention will becomeevident from the subsequent claims and from the following detaileddescription of the preferred embodiments of the invention, whichconstitutes one example and as such does not limit the extent of theprotection for the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

To simplify the understanding, the text contains references to encloseddrawings of figures, in which equivalent or similar parts have beenassigned the same referenced designation.

FIG. 1 shows schematically a sawing gear with constituent parts.

FIG. 2 shows the arrangement of a hydraulic coupling according to oneembodiment of the present invention.

FIGS. 3 and 4 show a section of the holder according to FIG. 1 includinga valve device according to one embodiment of the present invention inits working position and resting position respectively.

FIG. 5 shows a check valve device according to one embodiment of thepresent invention.

FIGS. 6 and 7 show a section of the holder according to FIG. 1 includinga valve device according to an alternative embodiment of the presentinvention in its working position and resting position respectively.

DETAILED DESCRIPTION OF THE INVENTION

The valve device according to the invention can either be arranged inits own stand-alone valve housing or it can be wholly or partiallyintegrated in another component of the hydraulic system or sawing gearto which the valve device is connected. In the embodiments describedhere, the valve device is integrated in a holder for the displaceablesaw bar that is included in the sawing gear.

FIG. 1 is a schematic illustration of sawing gear containing a holder 1,from which extend two parallel forks. The forks delineate a groove or aspace between them intended to be taken up by a steering component 4permanently attached to the saw bar 2. The steering component 4 has anessentially T-shaped cross-sectional profile and, together with the sawbar, forms what is essentially an H-shaped end piece whose narrow middlesection is intended to occupy the groove between the forks. Thisaccomplishes a connection of the saw bar to the holder that isdisplaceable in the direction of travel of the saw bar.

The displacement thus allows the tightness of a saw chain 3 runningaround the saw bar to be adjusted. The displacement is accomplished bymeans of a device that stretches the saw chain by acting between theholder and the saw bar. In the present embodiment, the chain stretchingdevice includes a hydraulic piston 5 arranged in the holder 1 and activein the direction of the displacement of the saw bar. This hydraulicpiston is referred to below as the tightening piston. The tighteningpiston 5 is arranged in the groove and applies a force against the sawbar's steering component 4 when the tightening piston is subjected tohydraulic pressure, at which it has the effect of displacing the saw barin the direction away from the holder, where, in fact, the driving wheelof the saw chain is arranged.

The tightening piston receives its pressure setting from the hydraulicsystem, by which a continuous and even stretching of the saw chain isaccomplished.

Our valve device 6 according to the invention is arranged connected tothe tightening piston 5 and, in the present embodiment, includes acylindrical space 7 and a displaceable piston device 8 that is adaptedto fit and move in the space, see FIGS. 3 and 4. Space 7 has openings infront of piston device 8; one is for connection/line 9 of the hydraulicsystem, referred to below as the inlet, and another is the connection 10to the tightening piston 5, referred to below as the outlet.

Outlet 10 is arranged with a check valve function, for example, in theform of a flap 11 that is moveable between an open position (see FIG. 4)that allows hydraulic fluid to flow freely between the outlet 10 and theinlet 9, and an active check valve position (see FIG. 3) that allowshydraulic fluid to flow in the direction from inlet 9 through outlet 10to apply pressure to the chamber 5′ behind the tightening piston 5 butwhich prevents liquid to flow in the opposite direction.

In the space, piston device 8 is under tension from a spring in thedirection towards the openings and has an extended projection 12 in theforward direction. When the hydraulic system is not pressurised, theflap device 11 comes under the influence of the extended projection 12of the spring piston, by which the projection 12 causes the flap to takeup its open position.

When the hydraulic system is pressurised, the hydraulic pressureincreases simultaneously in space 7 and chamber 5′ behind the tighteningpiston 5 since the hydraulic fluid can flow freely through outlet 10.The increasing pressure causes piston device 8 in the space to bedisplaced in a backwards direction against the tension of the spring andthus withdraw projection 12 from flap 11, which is then allowed to takeup its active check valve position still permitting apressure-increasing flow of fluid through outlet 10.

If, during work, a powerful load should occur to push the saw bar backtowards the holder, i.e. if a forceful increase in pressure occurs inchamber 5′ external to flap 11 in relation to the pressure in space 7,flap 11 will prevent the release of fluid through outlet 10 into space7. In this way, the saw chain retains its tension despite momentary peakloads.

When a saw chain 3 is to be fitted, the hydraulic system isde-pressurised by, for example, switching off the machine that powersthe hydraulic system and that may also support the sawing gear. Thepressure in chamber 7 falls and eventually reaches a level that can beovercome by the pro-sprung piston device, whose extended projection 12then applies a force to flap 11 that opens the check valve so that thepressure behind outlet 10 is released when the fluid is once againallowed to flow back through the outlet and into chamber 7. In this way,the saw bar can be pressed back into the holder by hand and a new chainfitted or an old one removed without the need for any great exertion orspecial tools.

The holder also includes clamping piston devices 13 arranged across thedirection of travel of the saw bar and that are also connected to ahydraulic system, preferably the same hydraulic system as the tighteningpiston. The clamping piston devices 13 must also be released when thesaw chain is to be replaced. It is thus advantageous if even these canbe equipped with a valve device according to the present invention,which in the present embodiment is preferably connected to the samevalve device as the tightening piston. This can be achieved, forexample, by channels in the holder between the different pistons 5 and13.

With this latter arrangement of connections, the system can be balanced,for example, by means of flow reducing restrictions in the differentlines that connect the pistons. In this way, for example, apre-determined flow restriction in the flow channel connections toclamping pistons 13 can be accomplished by the tightening piston 5tightening the saw chain before the hydraulic pressure presses the clamppistons 13 to its full extent.

In the present embodiment, space 7 consists of a hole bored in holder 1.The hole is threaded to a pre-determined depth so that a screw or a plugcan be used to close-off the hole.

Piston device 8 displays a cylindrical part that is adapted to the sidewalls of the hole. To ensure the tightness between the piston and thewall, at least one groove is also arranged around the piston'scylindrical section for a seal, e.g. an O-ring or piston ring orsimilar. The said projection 12 for opening the check valve is locatedat the front of the piston device. This projection should be designed sothat it does not hinder the fluid from flowing past when it projectsforward and opens the check valve. The projection 12 should, therefore,have a cross-section that is significantly less than the diameter of thehole and that is less than, or at least equal to, the opening it createsby opening the check valve. When piston 8 is in position in the hole, aspring of suitable type, e.g. a spiral spring, is inserted before thescrew is threaded into the hole to close off the valve device.

In the present embodiment, the flap device 11 contains a plate-like flap21 that moves between two stop positions 22 and 23. In the first stopposition 22, flap 21 bears on and seals off the flow path from outlet 10to space 7. In the second stop position 23, the flap bears on outletpath 23 during the build-up of open passages for the flow through offluid. These openings, through which fluid can flow in the secondposition, can, for example, be made in the actual flap to the side ofthat part of the flap surface that closes off the flowpath, or theflap's forward limitations 23 can be made shoulder-like to leave a gapbetween where the fluid can flow when the flap lies against them, or bysome other solution applicable for the flaps. The flap device can alsocontain a flap that is arranged to swing at a wall in outlet 10.

It should also be realised that other types of check valve that in asimilar manner can be opened by the external influence of the pistondevice, such as spring loaded ball valves, etc., should be interpretedas being equivalent to the flap device described here. The flap devicein the embodiment described above has the advantage of occupying a smallspace, which means that the physical distance between the tighteningpiston's pressure space 5′ and space 7 can be kept short. In this way,an exceptionally compact solution is achieved.

In addition, the flap device can be arranged with a light spring thatensures that the flap takes up a check valve location bearing on stopposition 22 when not under the influence of an external pressure.

Furthermore, it should also be realised that the said cylindrical space7 and the piston device 8 that is accordingly designed to fit and movein that space do not necessarily need to have a circular cross-section,but can have other shapes that are also suitable for the currentapplication, such as oval, rectangular, etc.

In an alternative embodiment shown schematically in FIGS. 6 and 7, thispiston device's adjustment of the returning fluid can be arranged inanother manner. In this case, a return flow passage 31 has been arrangedfrom a location behind the check valve, which here can be any checkvalve known to a specialised skilled person, to the cylindrically shapedspace. The return flow passage opens into the cylindrically shaped spacein a position that, with the piston device in its rear location (seeFIG. 7) is closed-off by the piston device's wall, and which, with thepiston device in its forward location (see FIG. 6), is situated oppositeto a through flow passage 32 running through the piston device andfurther on to a line 33 that either is connected to the pressure line 9of the hydraulic system, a pressure-free return line direct to a tank,to a separate tank, directly out into the surroundings, or to anothersuitable place.

What is claimed is:
 1. A valve device for automatic adjustment of thefluid pressure to a hydraulic tightening piston is a sawing gear betweena holder and a saw bar displaceable in a longitudinal direction relativeto the holder for adjusting the tightness of a saw chain running aroundthe saw bar and a wheel positioned on the holder, the tightening pistonbeing connected to a hydraulic system via the valve device, the valvedevice includes a cylindrically-shaped space and a displaceable pistondimensioned to fit and move between forward and rearward stop positionswithin the space, the improvement characterized in that the space infront of the piston has first and second opening connections, said firstopening connection or inlet to the hydraulic system and said secondopening connection or outlet to the tightening piston, an openable checkvalve being positioned in the outlet such that said check valve allowsfluid to flow only in one direction from the cylindrically-shaped spaceto the tightening piston, the displaceable piston being under tension bya spring and urged in a forward direction towards the outlet and theforward stop position causing the check valve to move to an openposition, whereby the displaceable position, when the hydraulic systemis in a non-pressurized state, is caused by the influence of the springto its forward stop position opening the check valve, and when thehydraulic system is in a pressurized state, is pushed back by thepressure in said cylindrically-shaped space to its rearward positionwhich permits the check valve to close preventing hydraulic fluid fromflowing from the tightening piston and into the cylindrically-shapedspace.
 2. The valve device according to claim 1, wherein the valvedevice is positioned in the holder.
 3. The valve device according toclaim 1, wherein the outlet is connected partly to the tightening pistonand partly to several saw bar-stabilizing clamping pistons positioned inthe holder to adjust the saw bar.
 4. The valve device according to claim1, wherein the check valve has a circular flap with a rear stop positionand a forward stop position, whereby the flap is configured to sealtightly against the rear stop position when subjected to anover-pressure or a jolt of pressure from the tightening piston, and whenpressed against the forward stop position permits fluid to flow betweenthe tightening piston and the cylindrically-shaped space.
 5. The valvedevice according to claim 1, wherein the check valve positioned in theoutlet is positioned between the cylindrically-shaped space and thetightening piston, and positioned symmetrically with regard to thecentral axis of the cylindrically-shaped space.
 6. The valve deviceaccording to claim 5, wherein the opening to the inlet is disposed in aside wall of the cylindrically-shaped space.
 7. The valve deviceaccording to claim 1, wherein the displaceable piston includes aprojection extending in a direction pointing towards the check valve, apressure spring behind the displaceable piston, and a seal between thedisplaceable piston and the wall of the cylindrically-shaped spacethrough which the projection, when the displaceable piston is in itsforward stop position, forces the check valve into its open position toallow free flow of fluid.
 8. The valve device according to claim 1,wherein a return flow passage extends from a location between the checkvalve and the tightening piston to the cylindrically-shaped space wherethe return flow passage opens in the wall of the cylindrically-shapedspace and that, with the displaceable piston in its rearward stopposition, is closed-off by the piston wall, and which, with thedisplaceable piston in its forward stop position, opens opposite to athrough-flow passage running through the piston and further to a linethat is connected to the hydraulic system.